Mesoporous materials, such as mesoporous silica and other metal oxides, are of great interest for application in numerous fields. For example, these materials are useful for separation and catalysis applications, and are also attractive candidates for low K dielectric applications. Literature reports indicate these structures can exhibit dielectric constants that can range between 1.3 and 2.1, making them desirable to the microelectronics industry.
To date, most mesoporous metal oxide materials have been prepared using aqueous media. For example, low K dielectric films are typically prepared by spin-casting a water/alcohol solution onto a substrate. These solutions contain a surfactant and a precursor, which develop mesoscale structure through a coordinated self-assembly, usually during evaporation of the solvent. As the solvent evaporates, the surfactant self-assembles into a template to which the precursor simultaneously associates. Removal of the template yields the mesoporous material. This approach is exemplified in U.S. Pat. No. 5,858,457, granted to Brinker.
D. A. Loy et al. (Chem. Mater., 9, 2264, 1997) describes silica aerogels that are prepared by condensation of tetramethoxysilane in supercritical CO2. Formic acid is used as a condensing agent, because agents commonly used for initiating the condensation of silica from a silicon alkoxide, such as H2O, typically have low solubility in CO2.
Porous metal oxides can also be prepared through a casting process using activated carbon as a support material (see, e.g., Wakayama and co-workers (Chem. Mater., 13, 2392 (2001) and Chem. Mater., 12, 756 (2000)). In this method, the activated carbon structures are coated by contact with CO2 solutions of metal alkoxide precursors, and precipitation is induced. The metal oxide precipitates onto the activated carbon structures, conforming to the structures' shape.